Well as far as I know, waze will only know about traffic/slow roads.
If either, a) its always slow at the time, and wazers have travelled through it, or b) There are active wazers on the road at the time.
With regards to re-routing, as I understand it, the app will effectively check your route periodically to see if there are any issues. What I am not sure of, is if this is prompted by traffic on your route or if it just requests the same route every 5 minutes and provides any updates if the routing servers provide a different one, some of this information is kept "classified" shall we say to ensure wazes product secrets. But hopefully one of the champs may know.
Now as you said in the first case there were users reporting stuff, but was there any in the "traffic" if so we know that wasn't the issue, I've seen this before and if I request the route manually it does change, so it may be a client issue. Of course other issues can be if you have no network, the client will not be able to check/update your route. If there are issues with the routing server.
Otherwise its possible despite users on the road, there may have been none in the traffic so waze just didn't know. Some people when they see the traffic will take their own route - As you did, however in this case waze will no longer know the road is slow. To work effectively, waze always requires at least one person to "take one for the team"
Perhaps one of the champs can comment though as they know far more...